Process for producing 2-ethylheptanoic acid

ABSTRACT

Process for producing 2-ethylheptanoic acid.

The present invention relates to a process for producing2-ethylheptanoic acid.

2-Ethylheptanoic acid is known per se. It has properties which make itpredestined as a replacement for the 2-ethylhexanoic acid used hithertoon a large scale.

Moreover, the plasticizers produced with 2-ethylheptanoic acid havelower volatility on account of the higher carbon number.

Hitherto, however, the replacement of 2-ethylhexanoic acid by2-ethylheptanoic acid has not been successful since no productionprocess is known which is able to produce 2-ethylheptanoic acid in highyields, cost-effectively and in a manner suitable for the industrialscale.

Prior art is a production process which is described in WO 02/16301.Here, the lactone 2-ethylidene-6-hepten-5-olide is cleaved to give2-ethylidenehexenoic acid and then hydrogenated to give 2-ethylheptanoicacid. The starting material 2-ethylidene-6-hepten-5-olide is producedcost-effectively in a known process from two molecules of 1,3-butadieneand one molecule of carbon dioxide (the process is described in Behr andBecker, Dalton Trans. 2006, 4607-4613).

The key step of the process described therein is the cleavage of the2-ethylidene-6-hepten-5-olide, the hydrogenating cleavage of the lactonebeing catalyzed with the help of “single site” complexes of metals ofsub-group 8 which have been modified with phosphine ligands. The processis carried out by means of a homogeneously dissolved catalyst.Alternatively, the immobilization of the “single site” catalyst on asolid support and also the use of a liquid-liquid two-phase system inwhich one phase consists of water are proposed as reaction control. Inthe case of the immobilization of the catalyst on a solid support, thephosphine ligands are modified such that they can be bonded to thesupport. When using a liquid-liquid two-phase system in which one phaseconsists of water, the phosphine ligands are modified with groups whicheffect solubility in water.

Disadvantages of the process described in WO 02/16301 are

-   -   1. Complex production of the catalysts    -   2. High sensitivity of the catalysts to oxygen and water    -   3. Difficult catalyst separation in the case of a homogeneous        reaction procedure    -   4. Complex modification of the phosphine ligands in the case of        immobilization and/or continuous catalyst removal.

An object of the present invention consists in developing a process forproducing 2-ethylheptanoic acid which does not have one or more of theseaforementioned disadvantages and therefore constitutes an improvementcompared to this prior art.

DE 2 003 522 describes a hydrogenolytic cleavage of saturated estersover bifunctional catalysts based on transition metals on a support withacid functions, which, according to the invention, are either ionexchanger resins or crystalline, zeolitic solid-body acids. According tothis document, it is particularly advantageous to use a solid-body acidwhich has an “alkylation index” above 100. This index is defined as theamount (in mmol of alkylate) of propyltoluene which is formed per gramof the acid under standard conditions (100° C., propylene saturated) inone hour.

This process cannot be used for the large-scale cleavage of3,6-diethyltetrahydropyran-2-one because of the completelyunsatisfactory useful life of the catalyst.

A. Behr, V. A. Brehme/Journal of Molecular Catalysis A: Chemical 187(2002) 69-80 describes, on page 71, the problem of opening the ring of3,6-diethyltetrahydro-2H-pyran-2-one, which, apart from the ketofunction, has no other double bonds. The problem could not be solved inthe aforementioned article. Consequently, there was the preconceptiontowards 3,6-diethyltetrahydro-2H-pyran-2-one that for precisely thiscompound no ring-opening reaction can be carried out.

A further object of the invention was thus to overcome the preconceptionthat a ring-opening for that specific compound,3,6-diethyltetrahydropyran-2-one, is not possible, and to developexactly such a ring-opening reaction for precisely this compound whichcould also be transferred to an industrial scale.

The objects were achieved by a process for producing 2-ethylheptanoicacid, involving the following process step:

where process step c) is carried out with heterogeneous catalysis over abifunctional catalyst or over a mixture of at least two catalystcomponents, where the bifunctional catalyst or, in the case of themixture, one catalyst component has an acidically acting component, andthe acidically acting component comprises a solid oxidic acid which hasan alkylation index below 100.

Surprisingly, it has now been found that the cleavage of3,6-diethyltetrahydropyran-2-one proceeds considerably better with avery high useful life of the catalyst if, instead of a crystallinesolid, the solid-body acid used is an amorphous silica-alumina which hasan alkylation index below 100.

In a further embodiment of the invention, the3,6-diethyltetrahydro-2H-pyran-2-one is obtained by the upstream processstep b):

In a further embodiment of the invention, the2-ethylidene-6-hepten-5-olide is obtained by the upstream process stepa):

The production of 2-ethylheptanoic acid can thus take place frombutadiene and CO₂ by linking together the following process steps:

a. Telomerization of butadiene and CO₂ to give2-ethylidene-6-hepten-5-olide.

b. Hydrogenation of the 2-ethylidene-6-hepten-5-olide to give3,6-diethyltetrahydropyran-2-one, preferably over a heterogeneouscatalyst on a neutral support, for example palladium on alpha-alumina.

c. Hydrogenolytic cleavage of the 3,6-diethyltetrahydropyran-2-one togive 2-ethylheptanoic acid.

Consequently, it is now possible to produce 2-ethylheptanoic acid on anindustrial scale starting from butadiene and CO₂.

In a further embodiment of the invention, the solid oxidic acid inprocess step c) comprises a substance selected from: amorphoussilica-alumina, zirconium dioxide, titanium dioxide. Here, the amorphoussilica-alumina is preferred.

Process step c) of the process according to the invention is carried outwith heterogeneous catalysis over a catalyst which preferably comprisespalladium, platinum or nickel, with palladium being particularlypreferred.

The catalyst can be configured in technical terms as a bifunctionalcatalyst or can consist of a mixture of two catalyst components.Preference is given to using a bifunctional catalyst. It can be producedin such a way that the hydrogenating component is applied to theacidically acting component. This can be carried out by impregnating theacidically acting component with a solution of a salt of thehydrogenating component, or jointly precipitating salts of theacidically acting component and of the hydrogenating component, asdescribed in J. Hagen, Industrial Catalysis: A Practical Approach,Wiley-VCH, Weinheim, 2006. Preference is given to carrying out animpregnation.

Process step c) of the process according to the invention can be carriedout in liquid phase, in liquid phase plus gaseous hydrogen phase or inthe gas phase. It is preferably carried out in liquid phase with thepresence of a gaseous hydrogen phase. In the process step, an organicsolvent can be present or the reaction can be carried out in the absenceof a solvent. Should a solvent be present, paraffins or ethers, forexample, can be used as solvents. Preference is given to carrying outthe reaction in the absence of a solvent.

The hydrogenolysis of the lactone 3,6-diethyltetrahydro-2H-pyran-2-onepreferably takes place such that 2-ethylheptanoic acid in high yields isformed as product. The alcohols and diols which are often formed duringthe hydrogenolysis of lactones are undesired in the process according tothe invention, and so their formation should be minimized.

Process step c) of the process according to the invention can be carriedout at temperatures in the range from 25 to 400° C., preferably in therange from 150 to 350° C.

The pressure can be in the range from 10 to 200 bar, preferably in therange from 20 to 100 bar.

The addition of hydrogen to the reaction mixture can take place infinely divided form and in amounts such that the stoichiometric ratio ofhydrogen to the starting material 3,6-diethyltetrahydro-2H-pyran-2-oneis between 2 and 1. Preferably, the ratio is between 1.5 and 1.1. It isparticularly preferably between 1.2 and 1.1, since otherwise there isthe risk that the acid group is hydrogenated to give the alcohol, or theproduct is further cleaved hydrogenolytically.

The ratio of the mass of the feed stream into the reactor to the mass ofthe catalyst per hour of residence time [Mfeed/(Vcat*RT), whereRT=residence time], known to the person skilled in the art as WHSV(weight hourly space velocity), can, in the case of process step c) ofthe process according to the invention, be in the range from 0.1 to 20 h⁻¹, preferably in the range from 0.5 to 5 h⁻¹.

Process step c) can be preceded by the following process step b), inwhich the 3,6-diethyltetrahydro-2H-pyran-2-one is obtained:

The 3,6-diethyltetrahydro-2H-pyran-2-one obtained here can serve asstarting material in subsequent process step c).

Process step b) of a process according to the invention of thisembodiment consists of the hydrogenation of2-ethylidene-6-hepten-5-olide to give3,6-diethyltetrahydro-2H-pyran-2-one. This process step is preferablycarried out with heterogeneous catalysis. For example, the catalyst inprocess step b) comprises palladium, platinum or nickel. Mixtures ofthese metals can also be used.

The metals can be used with or without support materials. Should asupport material be used, then support materials selected from the groupactivated carbon, aluminium oxide, silicon oxide, titanium oxide,zirconium oxide or magnesium oxide can be used. Preference is given tousing activated carbon or aluminium oxide.

The hydrogenation of 2-ethylidene-6-hepten-5-olide can be carried out inliquid phase, in liquid phase plus gaseous hydrogen phase or in the gasphase. It is preferably carried out in liquid phase with the presence ofa gaseous hydrogen phase. In the process step, an organic solvent can bepresent, or the reaction can be carried out in the absence of a solvent.Should a solvent be present, lower alcohols, paraffins or ethers, forexample, can be used as solvent.

The reaction temperature is for example in the range from 0 to 100° C.,preferably in the range from 20 to 80° C., particularly preferably inthe range from 30 to 70° C.

The pressure is usually in the range from 2 to 50 bar, preferably in therange from 6 to 30 bar, particularly preferably in the range from 10 to25 bar.

Furthermore, process step c) or process step b) can be preceded by afurther process step, process step a). In this,2-ethylidene-6-hepten-5-olide is obtained by the following reaction:

The 2-ethylidene-6-hepten-5-olide obtained in this way can serve asstarting material for process step b).

The 2-ethylidene-6-hepten-5-olide is accessible in high yields in acost-effective manner by virtue of a telomerization reaction of twomolecules of 1,3-butadiene and one molecule of carbon dioxide. Theprocess is described inter alia in Behr and Becker, Dalton Trans. 2006,4607-4613.

In one embodiment of the process, the 2-ethylheptanoic acid obtained inprocess step c) is reacted in a further process step d) with an alcoholto give an ester. The alcohol can be selected, for example, from:

monohydric, dihydric, trihydric or tetrahydric alcohols, particularlypreferably dihydric or trihydric alcohols. Among the di- or trihydricalcohols, particular preference is given, for example, to ethyleneglycol, diethylene glycol, triethylene glycol, tetraethylene glycol,propylene glycol, dipropylene glycol, tripropylene glycol, neopentylglycol, isosorbide, isomannide, isoidide, furan-2,5-dihydroxymethanol,trimethylolpropane, glycerol.

Very particularly preferably, isosorbide and glycerol may be mentioned.

The ester obtained in this way can be used for example as plasticizer,in particular for PVC or PVB.

As well as being used as a starting material for plasticizers, the2-ethylheptanoic acid produced according to the invention can alsoadvantageously be used in metal salts for use as thermostabilizer andsiccative and also for producing lubricants and for producing peroxidiccompounds.

The examples below are intended to illustrate the invention in moredetail without limiting its implementation to the procedure specified inthe examples.

WORKING EXAMPLES Hydrogenation of 2-ethylidene-6-hepten-5-olide to give3,6-diethyltetrahydro-2H-pyran-2-one (Process step b))

The hydrogenation is carried out in a 3 liter steel autoclave withheating jacket, through which a heat-transfer oil (Marlotherm SH fromSasol Olefins & Surfactants GmbH) flowed. The catalyst used is 5 g of acoated catalyst with 0.5% palladium on alpha-aluminium oxide in beadform which is incorporated into the reactor in a cage such that themixed gas and liquid phase flows through it in an optimum manner.

152 g (1 mol) of 2-ethylidene-6-hepten-5-olide are introduced into thesteel autoclave and dissolved in 11 of tetrahydrofuran. The autoclave isthen closed. By injecting hydrogen, a pressure of 20 bar is established.The suspension is held under these conditions at 60° C. for 20 h. Thesystem is then decompressed, the liquid phase is drawn off, the solventis distilled off on a rotary evaporator and the product is purified byfractional distillation. The yield of3,6-diethyltetrahydro-2H-pyran-2-one was 145 g (93%).

Hydrogenolysis of 3,6-diethyltetrahydro-2H-pyran-2-one to give2-ethylheptanoic acid (Process step c))

The hydrogenolysis is carried out in a tubular reactor (steel 1.4571,internal dimensions 800×8 mm) with a heating jacket, through which aheat-transfer oil (Marlotherm SH from Sasol Olefins & Surfactants GmbH)flowed. The catalyst used is 50 g of pellets (ca. 90 ml) of amorphoussilica-alumina with 13% aluminium oxide content and a BET surface areaof 290 m²/g which have been impregnated with 2.0% palladium. Thereaction temperature is 270° C. The WHSV value based on3,6-diethyltetrahydro-2H-pyran-2-one is 0.8 kg/I/h based on the emptyreactor. In parallel to 3,6-diethyltetrahydro-2H-pyran-2-one, 15 I/h(STP) of hydrogen are fed into the feed of the reactor. The reactor isheld at a pressure of 30 bar using argon. The discharge from the reactoris analyzed by gas chromatography. The yield is 89% of 2-ethylheptanoicacid.

1. A process for producing 2-ethylheptanoic acid, the processcomprising: (i) reacting 3,6-diethyltetrahydro-2H-pyran-2-one of formula(I) with hydrogen to obtain 2-ethylheptanoic acid of formula (II) in thepresence of a heterogeneous catalyst,

wherein the heterogeneous catalyst is a bifunctional catalyst or acatalyst mixture of at least two catalyst components, the bifunctionalcatalyst or one catalyst component of the catalyst mixture has anacidically acting component, and the acidically acting componentcomprises a solid oxidic acid which has an alkylation index below 100.2. The process according to claim 1, further comprising: before saidreacting (i), (ii) reacting 2-ethylidene-6-hepten-5-olide of formula(III) with hydrogen in the presence of a catalyst to obtain3,6-diethyltetrahydro-2H-pyran-2-one:


3. The process according to claim 2, further comprising: before saidreacting (ii), (iii) reacting a compound of formula (IV) with carbondioxide in the presence of a catalyst to obtain2-ethylidene-6-hepten-5-olide:


4. The process according to one claim 1, wherein the solid oxidic acidin said reacting (i) comprises a substance selected from the groupconsisting of an amorphous silica-alumina, zirconium dioxide, andtitanium dioxide.
 5. The process according to claim 1, wherein theheterogeneous catalyst in said reacting (i) comprises palladium,platinum or nickel.
 6. The process according to claim 1, wherein saidreacting (i) is carried out in the absence of a solvent.
 7. The processaccording to claim 1, wherein said reacting (i) is carried out at atemperature of from 25° C. to 400° C.
 8. The process according to claim1, wherein said reacting (i) is carried out at a pressure of from 1 barto 200 bar.
 9. The process according to claim 1, wherein said reacting(i) is carried out at a mass ratio of a feed stream to the heterogeneouscatalyst per hour of residence time of from 0.1 to 20 h⁻¹.
 10. Theprocess according to claim 2, wherein the catalyst in said reacting (ii)comprises palladium, platinum or nickel.
 11. The process according toclaim 2, wherein said reacting (ii) is carried out at a temperature offrom 0° C. to 100° C.
 12. The process according to claim 2, wherein saidreacting (ii) is carried out at a pressure of from 2 bar to 50 bar. 13.A process for producing an ester, the process comprising: reacting2-ethylheptanoic acid with an alcohol to obtain an ester, wherein the2-ethylheptanoic acid is obtained by the process according to claim 1.14. (canceled)